From the prior art, heterodisperse anion exchangers of the poly(meth)acrylamide type are already known. These are a class of anion exchangers which can be used in practice for numerous different applications.
An important area of use of heterodisperse anion exchangers of the poly(meth)acrylamide type is water treatment technology, in which it is possible to remove anions, for example, chloride, sulphate or nitrate, and weak acids such as salicylic acid and carbonic acid; organic acids such as formic acid, acetic acid, citric acid, humic acids and others.
Currently, both gel-type and macroporous heterodisperse anion exchangers of the poly(meth)acrylamide type are used in decolorizing press juices from beets and sugar cane. In the course of the complex production process of sugar extraction, the press juices from the beets, preferably sugar beets, and the sugar cane discolor. Pigments, for example melanoidines and caramel colors are formed. U.S. Pat. No. 4,082,701 discloses the use heterodisperse anion exchangers of the poly(meth)acrylamide type, for decolorizing pigment solutions. Raw solutions of liquid sugar syrup or invert sugar syrup are also currently desalted using heterodisperse anion exchangers of the poly(meth)acrylamide type.
It is also known to use gel-type or macroporous heterodisperse anion exchangers of the poly(meth)acrylamide type for the removal of acids or acidic components from whey and fruit thin press juices.
A known process for preparing heterodisperse anion exchangers of the poly(meth)acrylamide type is aminolysis of crosslinked acrylic ester bead polymers with polyamines according to U.S. Pat. No. 2,675,359, CZ-A 169 356, DD 99 587 or U.S. Pat. No. 5,414,020.
The crosslinked (meth)acrylic ester resin bead polymers used for the aminolysis are prepared in the prior art as gel-type or macroporous resins. They are prepared in mixed polymerization by the suspension polymerization process. This produces heterodisperse bead polymers having a broad particle size distribution in the range from approximately 0.2 mm to approximately 1.2 mm.
The heterodisperse anion exchangers of the poly(meth)acrylamide type obtained after the subsequent aminolysis can be quaternized by alkylating agents. The reaction to be performed here to give strongly basic groups can be carried out in the range from 1 to 100%, that is to say completely. Customary alkylating agents are alkyl halides or aryl halides or mixtures of the two, for example chloromethane according to U.S. Pat. No. 4,082,701 or benzyl chloride.
In U.S. Pat. No. 2,675,359, gel-type and macroporous heterodisperse bead polymers based on a methylacrylate-divinylbenzene copolymer are reacted with diethylenetriamine.
DD 99 587 describes the preparation of solid-grain weakly basic heterodisperse anion exchangers based on polyacrylic esters. The grain solidity is achieved by means of the fact that, after the copolymer is reacted with the polyamine, the resin is treated with a water-miscible solvent which swells the resin to a lesser extent than water.
Suitable solvents used are, for example, methanol, ethanol, acetone or mixtures thereof. 99% of the beads are obtained without cracks or fissures.
Without the treatment with methanol, for example, 35% of the beads have cracks and fissures.
The heterodisperse anion exchangers of the poly(meth)acrylamide type, depending on the charged form of the resin, that is to say depending on the type of counter ion to the nitrogen, exhibit differing resin volumes. When converted from the chloride form to the free base form, the resin swells markedly. Conversely, it shrinks on conversion from the free base form to the chloride form. In the industrial use of these heterodisperse anion exchangers of the poly(meth)acrylamide type, therefore, charging and regeneration is associated in each case with shrinkage or swelling, respectively. In the course of long-term use, however, these heterodisperse anion exchangers are regenerated several hundred times. The shrinking and swelling operations occurring in the course of this stress the bead stability so greatly that a fraction of the beads develop cracks, finally even fracturing. Fragments are produced which lead to blockages in the service apparatus and the columns, and impede flow, which in turn leads to an increased pressure drop. In addition, the fragments contaminate the medium being treated, preferably water, and thus reduce the quality of the medium or the water.
The flow of water through a column packed with beads, however, is impeded not only by resin fragments, but also by fine polymer beads, if present. An increase in pressure drop occurs. Due to the particle size distribution of known heterodisperse anion exchangers of the poly(meth)acrylamide type, beads of differing diameters are present. The presence of such fine beads additionally increases the pressure drop.
Seidl et al., Chemicky prumysl, roc. 29/54 (1979) cis 9,470, studied the aminolysis reaction of crosslinked acrylic ester resins and found that, in addition to the acrylamide unit, free acrylic acid units are also formed. All acrylamide resins exhibit free acrylic acid units.
After completion of charging of heterodisperse anion exchangers of the poly(meth)acrylamide type with anions, therefore, the resin is regenerated with dilute sodium hydroxide solution in order to prepare it for new charging. Sodium hydroxide solution residues are washed out of the resin with water. In addition the carboxylate ion which results from treating the carboxylic acid group with sodium hydroxide solution is hydrolysed by the water washing. During production of the resins a low conductivity of the effluent water (wash water) from the resin is desired, since otherwise impure water is present. The goal is to achieve low conductivity using small amounts of wash water, since this can be regarded as a sign that only small amounts of weakly acidic groups remain.